In the continuing search for hydrocarbons in the earth, geophysicists seek methods for evaluating and interpreting the structure of the earth's subsurface formations as well as the effects of stratigraphy, lithology, and pore fluid content on geophysical data in order to relate such effects to the occurrence or presence of hydrocarbons. Determining the occurrence or presence of hydrocarbons influences the placement of wells for recovering the hydrocarbons. Seismic reflection data are traditionally acquired and processed for the purpose of imaging acoustic boundaries, seismic reflection events, in the subsurface. By way of example, exploration geophysicists have developed numerous techniques for imparting seismic wave energy into the earth's subsurface formations, recording the returning reflected seismic wave energy and processing the traces. Such seismic signals or traces contain a multiplicity of information for example frequency, amplitude, phase, etc., which can be related to geology, lithology or pore fluid content of the earth's subsurface formations. Such features of the seismic signals are generally referred to as instantaneous attributes. Additionally, interpretative techniques generally referred to as stratigraphic interpretative analysis have been developed for analyzing seismic data and for identifying and characterizing changes in lithology, geology and pore fluid content of the earth's subsurface formations from recurring patterns of instantaneous attributes associated with reflection events in seismic data. Exemplary of such focus are Quay et al. in U.S. Pat. No. 3,899,768 and Bodine in U.S. Pat. No. 4,779,237.
The seismic attribute most commonly displayed during the interpretation of both two-dimensional and three-dimensional seismic data is amplitude. This has come about for good reasons as amplitude distinguishes many of the more subtle features of the subsurface that interpreters wish to identify. For example, amplitude `bright spots` are commonly used as direct hydrocarbon indicators and the correlation of reflections is often defined by a characteristic amplitude response. However, amplitude is only an indirect indicator of fault breaks and dip direction as fault breaks cannot be confidently identified in the time slice domain, therefore amplitude is not as useful in distinguishing fault breaks and dip direction.
Two-dimensional and three-dimensional seismic data are used for interpretation but the use of three-dimensional seismic data continues to grow. Three-dimensional seismic data provides a more detailed structural and stratigraphic image of sub-surface reservoirs than can be obtained from two-dimensional data. The results have been increased hydrocarbon reserve estimates, cost savings from more accurate positioning of delineation and development wells, improved reservoir characterization leading to better simulation models, and the ability to more accurately predict future opportunities and problems during the subsequent production of a field. As an exploration tool, three-dimensional seismic data reduces drilling risks in structurally complex areas and lends itself to reservoir quality prediction in undrilled areas.
The principal advantage of three-dimensional over two-dimensional seismic data is that three-dimensional provides the interpreter with the ability to view seismic data in horizontal "map" form rather than being limited to vertical cross-section views. Using traditional two-dimensional methods of viewing vertical profiles, it is often difficult to get a clear and unbiased view of faults and stratigraphic features hidden in three-dimensional data. Although faults are often readily seen on individual vertical seismic cross-sections, multiple vertical cross-sections must be examined to determine the lateral extent of faulting. Stratigraphic changes are difficult to detect on vertical seismic lines because of the limited profile that they present. To avoid these issues, geoscientists have traditionally utilized two kinds of seismic map displays: amplitude time-slice and seismic horizon-slices. The amplitude time-slice is a horizontal plane, at a constant time, through the three-dimensional volume which displays the amplitude of the seismic data at that time without reference to a stratigraphic horizon. An advantage of the amplitude time-slice is that an interpreter can view geologic features in map form without having to first pick seismic events in the data. The amplitude time-slice is quite under-utilized because the amplitude time-slices can be difficult to interpret, even for experienced geoscientists. The main reason for the poor utilization of this information is that the use of amplitude as the display attribute compromises the resolution of the data to the extent that features cannot be precisely positioned.
The interpretation of features on time slices is currently restricted by the use of seismic amplitude as the display attribute. The problem with amplitude in the time slice dimension is that it is impossible to distinguish the direction of dip from a single time slice. In order to distinguish dip, an interpreter must currently view an inline or crossline, or scroll through a number of time slices to see in which direction the events move. For large structures this is not too restricting, however, for more detailed structural interpretation it is frustrating and tedious.
Even when amplitude is used as the display attribute, the time slice domain does yield a degree of structural information and spatial correlations over large areas, however, dip direction and fault breaks cannot be identified with any certainty. In particular, it is the uncertainty of correctly locating faults on time slices that has developed the `look at but don't interpret` philosophy of seismic interpreters towards time slices. Both of the problems of dip and fault identification are caused by one of the properties of amplitude, which is for any cycle on a single trace there are always two equal amplitude values. Dual amplitude values make it impossible to distinguish the direction of dip because the amplitude values yield two possible solutions to the dip direction (i.e., either up dip or down dip) which results in an error in the direction of dip of 180 degrees as the two possible solutions are opposite. Secondly, the dual amplitude values diminish the resolution of the data for the identification of faults.
Time-slices are more suitable than vertical profiles for detecting and following the lateral extent of faults and stratigraphic boundaries. However, interpretation is often complicated by the fact that time-slices can cut through different stratigraphic horizons. This problem can be avoided through the use of the horizon-slice, which is the set of seismic amplitudes associated with an interpreted horizon surface, generally at some consistent stratigraphic level. The fact that the horizon surface is at a consistent stratigraphic level means that the attribute extracted from the seismic data to be displayed can highlight subtle lateral changes in the stratigraphy, lithology and fluid content at that one stratigraphic level. The resulting attributes have proved useful for detecting subtle faults associated with the one horizon. The use of previously interpreted surfaces for subtle edge detection is the current state-of-the-art.
In spite of the fact that horizon-slices and associated attribute maps are more useful than amplitude time slices for following faults and stratigraphic features, they too have disadvantages. The geoscientist must "pick" a stratigraphic surface prior to generating the amplitude display, which can be difficult and time-consuming. This also imposes an interpretive bias on the data set and requires that the interpreter has already defined the fault framework that intersects with the horizon under consideration. The edge detection routines are, therefore, only used to identify subtle faults that were not previously interpreted and to highlight the interpreted faults. The other disadvantage of horizon slices is that the results can only be obtained on isolated surfaces in the three-dimensional volume and not on the whole volume.
Other attributes of seismic reflections besides amplitude may be calculated and displayed in a map view as well, including frequency and instantaneous phase. These attributes, however, do not currently form part of the traditional three-dimensional seismic interpretation process and are used more as quality control checks of the processing and migration of the three-dimensional seismic data, or for enhancing event continuity in areas of noisy data on vertical profiles.